It is generally thought that mammals share a common mechanism whereby the circadian timing system is entrained to the 24 hour day. In addition to synchronizing diverse behavorial, physiological and biochemical rhythms to the solar day, this system also serves to internally synchronize these rhythms to each other so that appropriate phase relationships are maintained. Most investigators who study these circadian rhythms agree that light-dark cycles are the most efficacious entraining agents, and a reasonably complete model has been developed which accounts for the characteristic responses of mammals to photic information. However, this non-parametric phase response mechanism has been evaluated only in nocturnal rodents and there are several reasons why this model may not apply to diurnal primates, including man. We propose to test this non-parametric model in a diurnal non-human primate, the squirrel monkey, Saimiri sciureus. Previous work with this species has shown that it is an excellent human surrogate with rhythmic patterns quite similar to man's. The circadian rhythms of activity, drinking and body temperature in unrestrained monkeys maintained in isolation in various lighting conditions will be used to test specific predictions of the model. The results of this study should clarify how primate circadian rhythms are entrained.